Verbs which require a post-Head string are called TRANSITIVE verbs. The post-Head string, in these examples, is called the DIRECT OBJECT.

In contrast, some verbs are never followed by a direct object:

Susan [VP smiled]
The professor [VP yawned]

These are known as INTRANSITIVE VERBS.

However, most verbs in English can be both transitive and intransitive, so it is perhaps more accurate to refer to transitive and intransitive uses of a verb. The following examples show the two uses of the same verb: